In a generator such as a d.c. motor, generally, a pair of brushes is arranged on a stator and a commutator is arranged on a rotor. In the rotor, a plurality of armature windings are connected to commutator segments of the commutator, respectively. The pair of brushes is sequentially pressed against the commutator segments to supply the armature windings with electric currents, thereby rotating the rotor.
However, the brushes and the commutator segments are intermittently and repeatedly in contact with each other. Therefore, contact surfaces between the brushes and commutator segments are likely to be electrically abraded by the intermittent conduction, in addition to mechanical abrasion by press-contacting. This electric abrasion results from a discharge phenomenon (sparks) which occurs while the brushes repeatedly contact with and separate from the commutator segments.
To reduce such a discharge phenomenon, it is known to use a laminated brush constructed of two layers having different contents of copper against graphite, as disclosed in Japanese Unexamined Patent Publication No. 5-15114. Specifically, the brush has a high Cu-content section on a leading side and a low Cu-content section, which has a copper content lower than that of the high Cu-content section, on a trailing side with respect to the rotation direction of the commutator. By this construction, the life of brush is improved without deteriorating the performance of a rotary electric machine.
In the above brush, however, the performance such as conductivity is maintained by the high Cu-content section, and abrasion resistance is improved by the low Cu-content section. Therefore, it is difficult to further improve rectification (rectifying characteristic) and durability of the brush and the commutator.
In a brush disclosed in Japanese Unexamined Patent Publication No. 2003-100411, groups of protrusions are formed on a brush surface that contacts with commutator surfaces of a commutator. Further, the protrusions are formed at different phases against the rotation direction of the commutator. By this construction, initial vibration is suppressed, and durability of the brush is improved. Also, the brush is produced by adding 20 wt % or less boron nitride as lubricant into conductive metallic powder such as copper powder, graphite powder, and a binder. However, this brush is constructed by a single layer. Therefore, it is difficult to further improve rectification and durability.